1. Field of the Invention
This invention relates to the field of integrated circuits, and is more particularly directed to packages therefor, and still more particularly to ball-grid-array integrated circuit packages.
2. Description of the Relevant Art
Modern microprocessor-based data processing systems, particularly personal computers and computer workstations, are commonly upgradable in performance and capability. Typically, this upgrading is performed by replacing the originally installed microprocessor that serves as the central processing unit with a higher performance or more capable microprocessor. For example, one may upgrade a 386-based personal computer by removing the originally installed 386-type microprocessor and inserting, in its place, a 486-type microprocessor; by way of further example, one may upgrade the central processing unit by replacing the originally installed microprocessor with a microprocessor of the same type (e.g., 486-type) that can operate at a faster clock rate. Of course, other simple and minor adjustments of certain settings in the computer may also need to be made to accommodate the upgraded central processing unit. Additionally, the manufacturers of personal computers often install the microprocessor last to delay the associated financial commitment as much as possible and also because of certain import tax obligations.
Therefore, in the manufacture of upgradable personal computers, certain integrated circuits (e.g., at least the microprocessor) are installed into the system circuit board by way of sockets, rather than by being soldered directly to the circuit board, to facilitate removal and reinstallation of these components. Conventional sockets include sockets of the type that apply a mechanical force to pins of the integrated circuits so as to make contact thereto. However, as the number of terminals for microprocessors increase to more than 150 terminals, the total friction encountered in removing the microprocessor from such sockets becomes substantial, and may require a removal force of such magnitude as to crack or otherwise damage the board. While zero-insertion force sockets are also well known in the art, these sockets are generally quite expensive.
In addition, it is of course desirable to manufacture circuit boards with low-profile components, so that the overall height of the circuit board and its components is relatively low. This allows for the size of the overall system to be quite small, which is especially important for portable personal computers, such as those of the popular notebook size. However, conventional sockets, such as those described above, significantly raise the height of the circuit board and its components. The system designer and user are thus faced with the choice between reduced system size and upgradability in these circumstances.
By way of further background, ball-grid-array (BGA) packages have become popular in the art, particularly for packaging microprocessors. As is well known, the terminals of a BGA package are solder balls, formed on the underside of the package and attached to plated lands thereat. A solder reflow process is then used to attach the BGA component to a circuit board after its placement thereat; the heat of the reflow process melts the solder balls so that they will wet to lands on the circuit board, thus electrically and mechanically connecting the component thereto. The solder reflow process is especially useful as it can be done at relatively low temperatures, and the ability to surface-mount the BGA packaged component eliminates the requirement of plated-through holes in the system circuit board. In addition, BGA packages allow a large number of terminals to occupy a small board area, as the pitch of solder balls on the package can be quite small (e.g., on the order of 0.1 cm). The BGA package is also a very low profile package, and is thus especially suitable for small systems such as notebook-size personal computers and the like.
However, according to the current state of the art, the benefits of BGA packages are not available if the component is to be installed by way of a socket. Firstly, the low profile provided by the BGA package is lost, for the most part, when the height of the socket is to be added to the system. Secondly, the solder balls tend to provide poor performance as a mechanical connection (i.e., when not reflowed to provide a solder connection). This is because conventional solder compositions tend to migrate, or creep, under mechanical force. In addition, the mechanical force required to make electrical contact to a cold solder ball is quite high (e.g., on the order of one ounce), exacerbating the problem of solder creep under mechanical stress. This high contact force is required in order to overcome the native oxide that rapidly forms over the surface of conventional solder balls. Despite these barriers, sockets for making removable connection to solder balls on a BGA package are known in the art, as will now be described relative to certain examples.
FIGS. 1a and 1b illustrate an example of a conventional socket for BGA components, such as is available from Method Electronics, Inc. In this example, referring to FIG. 1a, BGA package 2 is converted into a pin-grid-array package (PGA), by reflow soldering of the BGA package 2 to pin board 4. As shown in the cross-section of FIG. 1b, pin board 4 solder connects each solder ball 3 of BGA package 2 to a corresponding pin 5 extending through pin board 4. The combination of BGA package 2 and pin board 4, so assembled, acts as a PGA package, and may now be inserted into conventional socket 6 that is soldered to the surface of system circuit board 8. As shown in detail in FIG. 1b, each of pins 5 of pin board 4 extend into and are received by pin sockets 7 in socket 6. Pin sockets 7 are each soldered to system circuit board 8, by way of solder joints 9 as shown in FIG. 1b. However, as is evident from FIG. 1b, the presence of both socket 6 and pin board 4, along with the height of reflowed solder balls 3, result in a relatively high profile combination package system. Furthermore, the cost of this package system with its additional parts and extra labor can be quite high, considering that two additional piece parts (pin board 4 and socket 6) are required.
Referring now to FIGS. 2a and 2b, another example of a conventional socketing arrangement for a BGA package, manufactured and sold by AMP, will now be described for purposes of further background. As shown in FIG. 2a in cross-section, BGA package 12 has solder balls 13 connected thereto, extending from the bottom surface thereof. Solder balls 13 are received by receptacles 16 in socket 14, which in turn is soldered to system circuit board 18 by way of solder joints 19. Referring to FIG. 2b the plan view of FIG. 2a, receptacles 16 are, in this example, configured as partially split rings, so that burrs 17 are present on the inner diameter of receptacles 16. Burrs 17 serve to cut into solder balls 13 when BGA package 12 is installed over socket 14, and thus provide both mechanical support and also electrical connection between circuit board 18 and solder balls 13. However, this socketing approach requires a socket thickness of at least the height of solder balls 13 (in their cold, non-reflowed state), and also suffers from the difficulties of high contact force and solder creeping under stress to which cold solder connections are vulnerable, as noted above.
By way of further background, it is known to make removable mechanical connections between integrated circuits having solder balls disposed thereon (e.g., according to the well-known "flip chip" technology) and copper pads having plated palladium dendrites formed thereon. This approach is believed to be used in connection with hybrid, or multi-chip, modules, where replacement of faulty integrated circuit chips is desired. In this example, the solder balls are mechanically forced onto the dendrites, which serve to grip the solder ball and mechanically hold the chip in place, as well as provide electrical connection thereto. Again, the difficulties associated with high contact force and creeping of the solder material are also believed to be present in this approach.